Rolled tissue products, as well as other rolled paper or nonwoven products, are typically perforated ("perfed") in order to facilitate the tearing off of the desired length of product for the intended use in a neat and undamaged fashion. In tissue products, the perforations facilitate easy removal of the required number of sheets. The perforations are normally provided in transverse perforation lines across the roll width, which are uniformly spaced in the machine direction of the roll. The lines of perforations comprise alternating bonds and perfs which are of uniform length and spacing. The perfs are typically rectangular slits or round holes with transverse orientation.
Perforating devices are well known in the papermaking art and are incorporated into almost all bathroom tissue and towel winders as well as other converting equipment in a typical tissue manufacturing and converting plant. These devices comprise a perforator roll, which holds a number of perf blades, and a stationary anvil head, which holds a number of anvil assemblies. The anvil assemblies are typically positioned helically (on a curve) on the stationary anvil head so as to keep all of the perforator blades from striking all of the anvils at the same time, thus minimizing the amount of vibration at the point of perforation. The perf blades themselves are typically 4.5 inches long, 0.875 inch wide, and from about 0.030 to 0.040 inch thick. The perf blades are typically specified by the length of the bonds (nonperforated segments of the perforation line) and the number of bonds per blade. Thus a typical 4.5 inches long perf blade might be designated as a 0.020 inch.times.40, indicating that its bonds were 0.020 inch long and that there were 40 such bonds on each 4.5 inches long perf blade. In the converting process a balance must be struck between having perforation lines that have sufficient bond strength to operate efficiently and without breaks on the converting equipment, and yet have low enough bond strength to provide easy and undamaged sheet detaching for the consumer. Nevertheless, in spite of efforts to achieve this proper balance, poor detaching has always been one of the major consumer complaints for rolled tissue products such as toilet tissue or kitchen towels.
Poor detaching usually manifests itself to the consumer as the incomplete removal of a sheet of tissue at the line of perforations. Usually the web will start to tear at the perforation line, but as the tear progresses across the line of perforation in the roll width, the web will start to tear longitudinally in the machine direction rather than transversely across the roll at the perforations. The result is typically the leaving of an "ear" or piece of sheet that had been detached at the far end of the roll from which the detaching had been initiated. Another fairly common problem is that the perf bond strength is too high, favoring good operation of the converting equipment, but when the sheet is detached, the web initially tears in some spot other than at the line of perforation. The leaving of "ears" is by far the more common problem of the two. The problems associated with poor detaching are normally worse in two-ply products than in one-ply products, and they are particularly bad in two-ply products that have poor attachment of the two plies, such as two-ply towels that are mechanically attached by embossing rather than being embossed and then glued together.
Hence there is a need to provide a perforated product that detaches more uniformly and completely at the perforations such that the detached sheet is in its whole undamaged form after detaching. There is also a need to maintain good operational efficiency of the converting equipment without the frequent "blow outs" or sheet breaks that can be caused by trying to operate converting equipment when the perforation line bonds are too weak.